1. Field of the Invention
The present invention relates to a method of performing beam compression of an antenna pattern, which can provide better performance in the beam compression process of an antenna pattern of a radar utilizing the multiplicative array principle.
2. Description of the Related Art
In general, a beam width is one of indexes representing the performance of an antenna pattern of a receiving antenna or other type antennas. A narrower beam width of an antenna pattern will give a better performance. However, there is a relationship of inverse proportion between a beam width and the size (length) of an antenna. Therefore, if the beam width is reduced, then the size of the antenna will be increased. Conversely, if the dimension of the antenna is reduced, then the beam width will be broadened.
For example, in an antenna for a radar system, if it is desired to double the ability or the resolution to discriminate objects, it is required to halve the beam width and thus it is required to double the size of the antenna. The doubling of the size leads to not only a larger occupied region but also various disadvantages such as an increase in the weight of the antenna and in the size of a structure for supporting the antenna. Conversely, if the size of an antenna is halved, then the beam width will be doubled and the discrimination ability will be degraded by a factor of two.
It is well known that there is such a conflicting relationship between a beam width and the size of an antenna. In most cases, an actual antenna has a limitation in the region it can occupy. Therefore, under these limited conditions, a certain degree of compromise associated with the beam width has to be made.
One known beam compression technique to alleviate the above-described problems is to reduce the beam width by means of multiplication of received signals of a plurality of antennas according to the multiplicative array principle. FIG. 1 is a schematic diagram illustrating a configuration of a radar system which can perform the beam compression in such a manner described above. In this figure, reference numeral 101 designates a main antenna such as an array antenna comprising a plurality of radiation elements which are equally spaced along a straight line. Reference numeral 102 designates a sub-antenna disposed apart from the main antenna 101 in the X-direction which is the direction of a beam-width to be reduced. Reference numeral 103 designates a transmission circuit which generates transmission power, which is in turn fed to the main antenna 101. Reference numeral 104 designates a multiplying circuit which performs multiplication between a received electric field signal received by the main antenna 101 and a received electric field signal received by the sub-antenna 102. Reference numeral 105 designates a rectifying circuit which provides an output signal only when the multiplied output signal from the multiplying circuit 104 is positive. In FIG. 1, the beam-axis direction is perpendicular to the X-Y plane or the plane of the drawing.
In the antenna system having such a configuration described above, electric field signals received by the respective antennas 101 and 102 are fed in the same phase to the multiplying circuit 104 so as to perform the multiplication on these signals. The multiplied signal is output via the rectifying circuit 105 only if the multiplication result is positive. As a result, a received electric field signal (electric field pattern) corresponding to the directional characteristic of the main antenna 101 such as that shown in FIG. 2 is multiplied by a received electric field signal (electric field pattern) corresponding to the directional characteristic of the sub-antenna 102 such as that shown in FIG. 3, thus providing an output signal (output pattern), such as that shown in FIG. 4, corresponding to the synthetic directional characteristic having a beam width .theta.wc which is reduced from the beam width .theta.w of the electric field pattern associated with the main antenna.
In the beam compression method of an antenna pattern according to the conventional technique based on the above-described multiplicative array principle, there is a problem that it is impossible, as a matter of course, to achieve a resolution better than that corresponding to the beam width .theta.wc of synthetic directional characteristic, shown in FIG. 4, obtained by performing the multiplication process on the received electric field signals.